Conventionally, as a strain sensor which detects strain in an axial direction, a strain sensor has been known which is disposed between a nut disposed in a movable manner in an axial direction of a feed screw shaft and a movable table fixed to the nut so as to detect strain acting between the nut and the movable table (see Japanese Unexamined Patent Application, Publication No. Hei5-138481, for example). For a joint of a robot on which loads in various axial directions act, in order to detect the loads in the axial directions which act on the joint with high accuracy, a strain cancellation mechanism is formed of a bridge circuit which uses a large number of strain sensors capable of detecting strain in the axial directions, and loads in axial directions other than a specific axial direction along which detection is expected are excluded.